An important problem for packet transfer over radio interfaces is the overhead caused by the packet headers. If a large number of relatively small packets are transferred, such as in interactive voice conversations, the overhead accounts for a significant part of the available bandwidth. Consequently, header compression protocols are usually supported in most radio interface standards.
One header compression technique is specified in the Internet Engineering Task Force (IETF) Request For Comments (RFC) 3095 where the compression scheme is denoted as Robust Header Compression (RoHC), see “RObust Header Compression (RoHC): Framework and four profiles: RTP, UDP, ESP and uncompressed”, IETF RFC3095. This compression scheme is designed to work well when used over links with high error rates and long-round trip times, as opposed to other existing compression schemes.
In RoHC the compressor and de-compressor operates using multiple states. The operation always starts in the lowest compression state and transitions are made gradually towards higher compression states. The mode of operation controls the logic of state transitions and what actions to perform in each state.
Generally, there are two different types of modes of operation; unidirectional and bidirectional. In unidirectional mode, packets are sent from compressor to de-compressor and thereby transitions between compressor states are performed without any knowledge about the de-compressor state, e.g. periodically. In turn bidirectional modes have feedback mechanisms for error recovery requests and acknowledgements of context updates from de-compressor back to compressor.
Bi-directional modes typically do not have as frequent transitions to low compression states as unidirectional mode and therefore they have (in some sense) better compression efficiency.
In certain cases however, such as, for example, Multimedia Broadcast Multicast Services (MBMS) of which mobile TV is one example, a fundamental difference compared to conventional mobile communication services is that MBMS is characterized by point-to-multipoint connections instead of point-to-point connections.
One consequence of point-to-multipoint communication is that there are multiple de-compressor machines (typically one in each terminal) and typically one compressor machine per service at the network side. Similarly, feedback channels between compressor(s) and de-compressors are undesirable (or infeasible) and therefore unidirectional mode of operation is the preferred choice.
In point-to multipoint connections a problem arises with conventional technology, since the number of involved de-compressor machines is constantly changing because some users join and leave the MBMS service every now and then. Hence, in order to create contexts for all de-compressors, the compressor must perform a transition to the lowest compression state whenever a new user has joined the service. In general, the arrival of a new user is unpredictable and therefore the compressor must perform these transitions in a very conservative manner. In reality the compressor may need to always assume worst-case situations with a large user population and short inter-arrival times. Although this type of design creates the necessary context for all de-compressors, the overall header compression efficiency is impaired due to frequent transitions to the lowest compression state.
Hence there exists a need for a more efficient transmission of data in point-to-multipoint connections.